This invention relates to a semiconductor device containing a network of resistors such that its resistance can be fine-adjusted and more particularly to such a semiconductor device comprising an improved trimming means for carrying out a fine adjustment of its resistance.
For providing an integrated circuit (IC), or a semiconductor device containing a resistor of which the resistance can be fine-adjustable so as to have a desired value, it has been known (as disclosed, for example, in Japanese Patent Publications Tokkai 5-235282 and 8-335674) to make use of a resistor network comprising a plurality of resistor units and a trimming means such that a fine adjustment, or a trimming process, can be carried out after its resistance is measured.
FIG. 8 shows an example of such a prior art IC 1 including a resistor network formed by connecting four resistor units Ra, Rb, Rc and Rd in series and trimming units 2, 3 and 4 connected in parallel respectively with resistor units Ra, Rb and Rc. Resistor unit Rd is designed to have a resistance close to a target resistance value, while resistor units Ra, Rb and Rc are designed to have small resistance values corresponding to the variations from the target resistance. The trimming units 2, 3 and 4 are each capable of being selectively in either a short-circuit (or closed) condition or a released (or open) condition and also of changing its condition at least once. At the time of a probe test after a wafer processing, the resistance between terminals A and B is normally measured and, if this measured resistance is within a specified allowable range, this wafer or a bare chip is directly sent to a next process. If the measured resistance is not within the allowable range, one or more of the trimming units 2, 3 and 4 are selected and their conditions are changed, depending on how far the measured value is off the allowable range. Thus, a trimming can be carried out so as to bring the final resistance value within a range between the resistance of resistor unit Rd and that obtained by adding thereto the resistance values of the other resistor units Ra, Rb and Rc. Those of wafers and bare chips, on which a trimming and other tests have been carried out, are directed to a later process and ICs are assembled through wire bonding and packaging steps. After ICs are thus completed, the trimming units 2, 3 and 4 become hidden inside the ICs, screened off from outside.
At the time of measuring the resistance, however, one cannot ignore the contact resistance between the probe pin and the pad because this contact resistance varies sensitively and unstably in a wide range of about 1.OMEGA. to 10.OMEGA., depending on the extent of oxidation, for example, on the contact surfaces of the tungsten probe pin and the aluminum pad which tend to form a layer of oxide on the surface as well as the scratches on the pad surface sustained by the tip of the probe pin. As a result, the adjustment of resistance is not complete by a trimming process alone based on a probe test, and many IC products fail to pass the final test and are discarded.
On the other hand, there is an increased demand for accurate electronic components, and a trimming process is frequently required to be carried out even on resistors contained inside an IC. As the level of integration is improved and ICs come to have many pins, the number of pins on probe cards is also increased and the probe pins become thinner. Thus, the contact resistance cannot be expected to become stable, and the unstable contact resistance means a lower yield of products.
In order to make it possible to fine-adjust the resistance of an IC even after it has been assembled, one might consider an IC shown in FIG. 9 at 1a, having fuses 5, 6 and 7 instead of the trimming units 2, 3 and 4 and pairs of pads 5a and 5b, 6a and 6b, and 7a and 7b each connected to both ends of the associated one of the trimming units 5, 6 and 7, as shown. A fine adjustment of such an IC is possible by supplying a fusing current from an external source to cut an appropriate fuse (or fuses) through an IC pin (not shown) connected to each of the pads. With an IC thus structured, however, the fuses for fine-adjustments remain connected to the pins even after the IC is completed and hence external effects on the pin can directly influence the fuses. Thus, its resistance may unexpectedly change due to a damage to the fuse caused electrostatically, such as by an external surge current. Moreover, since two pads or one pad and a pin are required for each fuse, there are a large total number of pads and pins which are used only for the adjustment of the resistance.
Such an increase in the numbers of pads and pins is not desirable especially where a high degree of integration is required.